Method for regulating heat output from an oxidizing fluidized bed

ABSTRACT

Gas is injected into a localized area in an oxidizing fluidized bed boiler to fluidize a portion of the bed. By varying the relative pressures and flow rates of the locally injected gas supply and the main fluidizing gas supply a selected portion of the bed is fluidized, while the remainder of the bed remains in a static condition. Fuel distribution and combustion are restricted to the fluidized portion of the bed, making possible a wide range of thermal output variation.

United States Patent John W. Bishop Alexandria, Va.

Sept. 24, 1969 Feb. 23, 1971 the United States of America as representedby the Secretary of the Interior lnventor Appl. No. Filed PatentedAssignee METHOD FOR REGULATING HEAT OUTPUT FROM AN OXIDIZING FLUIDIZEDBED 11 Claims, 1 Drawing Fig.

U.S. Cl 110/28, 122/4 Int. Cl F23d 19/02 Field of Search 110/28; 122/4LOCAL SUPPLY [56] References Cited UNITED STATES PATENTS 2,871,0041/1959 Gorin 110/28X 2,976,853 3/1961 Hunter et a1 122/4 3,397,6578/1968 Tada 110/28X Primary Examiner Edward G. Favors Attorneys- ErnestS. Cohen and Albert A. Kashinski ABSTRACT: Gas is injected into alocalized area in an oxidizing fluidized bed boiler to fluidize aportion of the bed. By varying the relative pressures and flow rates ofthe locally injected gas supply and the main fluidizing gas supply aselected portion of the bed is fluidized, while the remainder of the bedremains in a static condition. Fuel distribution and combustion arerestricted to the fluidized portion of the bed, making possible a widerange of thermal output variation.

MAIN GAS SUPPLY PATENTEDZIFEBZSIHYI LOCAL GAS SUPPLY MAIN GAS SUPPLY MVENTOR JOHN M. BISHOP ATTORNEYS METHOD FOR REGULATING HEAT OUTPUT FROMAN OXIDIZING FLUIDIZED BED BACKGROUND OF THE INVENTION This inventionrelates generally to fluidized beds, and more particularly to a methodfor regulating the rate of fuel consumption and heat output fromoxidizing fluidized beds.

Simply stated, a fluidized bed consists of a mass of discrete particlessuspended within a walled enclosure by a flowing stream of fluid, whichenters the enclosure through a porous bottom surface. As the fluid,which may be a liquid or gas, passes upward through the bed, individualparticles are suspended in the stream and disengaged from one another.Mobility of the particles within the bed is enhanced, so that the massof fluidized particles resembles a high viscosity liquid. The fluidizedparticulate suspension obtained by this process is useful in a widevariety of chemical and industrial processes.

Recently developed fluidized bed boilers employ oxidizing fluidized bedsfor heat generation. A bed of inert granular material is supported incontact with heat exchange surfaces by a stream of air within theboiler. When the fluidized bed is heated above a critical ignitiontemperature and is supplied with a mixture of air and coal, intensecombustion occurs within the bed. High heat releases,and heat transferdirect from the bed material to the heat exchange surfaces enhance theefficiency of the boiler,reducing the boiler dimensions required for aspecified thermal output, in comparison with more conventional boilerdesigns.

Although heat exchange efficiency is inherent in the operation ofoxidizing fluidized bed boilers, a relatively narrow range for variationof fuel consumption and heat output in each individual boiler hasimpeded their general commercial acceptance. Previously, it wasnecessary. to operate the oxidizing fluidized bed near the maximumoutput, or, alternatively, to completely shut the bed down. Thisall-or-nothing operation made inefficient use of fuel and lacked theoutput flexibility available from conventional boilers. Partitionedbeds, the sections of which were operable individually or in groups,were investigated in attempts to provide additional discrete steps inthe operation of fluidized bed boilers. However, the turndown ratiocontinued to be limited by the operational limits of each individual bedsection.

SUMMARY or THE INVENTION This invention is a method for improving theturndown ratio of oxidizing fluidized beds. It is based upon thedetermination that selective fluidization of a portion of a bed can beachieved under carefully controlled operating conditions. By varying thesize of the fluidized portion, the amount of combustion within the bedcan be regulated over a wide range.

When a fluidizing gas, such as air, is directed upward from adistribution grid and through a particulate bed, the bed remains in astatic condition until the gas velocity is sufficient to fluidize thesmallest, lightest fraction'of the particulate. At this gas velocity thesmaller particles rise to the top of the bed in a fluidized state, whilethe larger particles remain, in static condition below them. If alocalized stream of gas, such as air or fluegas, is injected incombination with the main fluidizing gas supply at a point within thestatic lower portion of the bed, with sufficient pressure and flow rateto overcome the static resistance of the bed, fluidization of the largerparticles results in the area of localized gas injection. Solid fuelparticles injected into the bed at a point near the localized gas streamcan then penetrate the fluidized upper and lower portions of the bed toprovide discrete areas of combustion, regardless of the overall extentof bed fluidity. The fuel particles penetrate only the fluidizedportion, and not the staticportion of the bed. By maintaining the bedbetween the minimum ignition temperature of the fuel and theagglomeration temperature of the bed, continuing combustion within thelocally fluidized portion is possible.

Combination of a localized gas injection with a main fluidizing gassupply may be used in this way to obtain a wide variation in the heatoutput from the bed. In addition, with suffcient pressure and flow ratethe localized gas supply alone is sufficient, in itself, to fluidize asmall portion of the particulate bed, so that the boiler output iscontinuously variable between the minimum limit imposed by localfluidization and the maximum limit.

Three distinct fluidization conditions represent the possible range ofoperation in a fluidized bed regulate'dby the method of this invention.During minimum load, only that portion of the bed fluidized by thelocalized gas supply is available for active heat production andtransmission to imbedded heat exchange surfaces. During an intermediateload condition, the portion of the bed which is locally fluidizedinteracts with the upper portion of the bed which is fluidized by themain fluidizing gas supply. In this condition, fuel particlestransported into the bed near the local injection point readily diffusethrough the combined fluidized portions of the bed, but do not penetratethe static portion of the bed. Oxidizing bed temperature is reached onlyin the fluidized portions of the bed. Under full load condition, theentire bed is fluidized by the main gas supply, and the injected fuel isdistributed uniformly and oxidized throughout the bed. This lattercondition is the state of bed operation achieved by the prior art.

Therefore, one object of this invention is a method for obtaining a widerange of thermal output variation in an oxidizing fluidized bed.

Another object of this invention is increased operating efficiency andflexibility in oxidizing fluidized bed boilers by enabling theiroperation over a wide range of thermal output.

Still another object of this invention is to increase the turndown ratioof oxidizing fluidized bedboilers.

These and other objects of the invention will become more apparent withreference to the following specification and drawing.

The sole FIG. is a partial section of an oxidizing fluidized bed boiler.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the sole FIG. an oxidizingfluidized bed boiler 10 is shown. The boiler 10 incorporates designdetails which are well known in the prior art and includes an enclosure12 with vertical sidewalls 14 and a horizontal base 16. Within theenclosure 12, fuel is mixed with air and burned. I-leat generated bythis process is transferred through heat exchange surfaces 18 to aworking fluid, flowing in a conduit 20 within the boiler. Working fluidenters and leaves the conduit 20 through supply pipes 22 on the exteriorof the boiler, passing to its ultimate use in a heated state.

The combustion zone 24 of the boiler 10 is a bed 26 of different sizedinert particles which can be suspended in a fluidized state by a streamof gas from a blower 28. Gas, such as air or a mixture of air and othergases, from the blower 28 enters the boiler 10 near the base 16 of theenclosure 12. This main fluidizing gas supply flows upward into the bed26 through a perforated distribution grid 30, which is spaced upwardfrom the base 16. In the distribution grid 30 there is an array ofspaced perforations 32 which distribute the main gas supply in an evenstream throughout the horizontal cross section of the bed 26. When thegas is injected into the bed with sufficient pressure and flow rate,individual particles in the bed are disengaged from one another andsuspended in a fluidized state.

Depending upon their relative sizes, particles in the bed 26 arefluidized by different pressures and flow rates of the fluidizing gas.Smaller particles are fluidized by lesser pressures and flow rates thanlarger particles, and so the smaller particles drift to the top of thebed. When the pressure and flow rate is insufficient to fluidize largeparticles the lower part of the bed, the large particles remain in astatic condition while the smaller particles may form a fluidized layeron the top. In this way it is possible to fluidize a fractional portionof the bed with the main fluidizing gas supply.

Fuel enters the boiler through an injection port 34 adjacent to thedistribution grid 30 at the bottom of the bed 26. The fuel is preferablya particulate solid, such as crushed coal, which is mechanically orpneumatically injected into the bed. For pneumatic injection the fuel ismixed with a local gas supply, such as air or fluegas, at the junctionof a fuel supply pipe 36 and a gas supply pipe 38. When the entire bedis in a fluidized state, the injected fuel spreads throughout the volumeof the bed with relatively uniform distribution. By maintaining the bedtemperature above the ignition temperature of the fuel, reaction of thefuel and air mixture results in uniform combustion.

lt has been found that if sufficient pressure and flow rate aremaintained, the local gas supply acting alone is sufficient to fluidizea fractional portion of the bed 26. The required pressures and flow ratedepend upon design characteristics of the bed and boiler, and are easilydetermined experimentally by increasing the input volume of the localgas supply until partial fluidization occurs. When the localized supplyis injected with sufficient pressure and flow rate, fluidization in thearea immediately above the injection port 34 results, as depicted by theline A-A in the HO. Incoming fuel particles penetrate only the fluidizedportion of the bed, and active oxidation is limited to the portion ofthe bed to the left of line A-A. The only direct heat transfer from thebed 26 to the heat exchange surface 18 takes place in the fluidizedportion. The balance of the bed, to the right of line A-A remainsrelatively cool and in static condition, with little heat transfer intothe heat exchange surfaces. By designing the heat exchange surfaces, sothat they occupy only the space to the right of line A-A, operation ofthe bed can be maintained without substantial heat transfer to theworking fluid.

The effects of the main fluidizing gas supply can be combined with thoseof the local gas supply to achieve varying states of fluidization withinthe bed 26. By increasing gas flow through the distribution grid 30, afractional upper portion of the bed can be fluidized, as explainedabove. The fractional portions affected by each gas supply combine toform a larger area of fluidization than occurs with either gas supplyacting alone, as shown by the area above and to the left of line A-B inthe FIG. The active area for heat transfer from the bed to the heatexchange surface 18 is thus increased.

By varying the relative pressures and flow rates of the main and localgas supplies, other states of partial fluidization are possible. Eithergas supply may be reduced to a pressure which is insufficient alone toproduce fluidization, but in combination with the other gas supply issufficient for partial fluidization. It is only necessary that thecombined pressures and flow rates be sufficient to fluidize a portion ofthe bed adjacent to the fuel injection port so that there is a path forinjection of fuel into the active area of the bed. When these conditionsexist, the static portion of the bed remains relatively impervious toincursions by the fuel and localized gas stream.

For full load operation the pressure and flow rate of the main gassupply from blower 28 are increased in an amount sufficient to fluidizethe entire bed, as practiced by the prior art. Additional temperaturecontrol at this and lesser load conditions may be achieved by combiningair and recirculated fluegas in the main fluidizing gas supply toregulate the oxidation rate within the bed 26.

While the sizes of the bed particles and the type and size of fuelemployed will vary according to well known fluidized bed designparameters, experiment has shown that a bed of coal ash particles in therange of /5 inch by 20 mesh is suitable for the method of thisinvention. Coal in the range of Y4 inch to inch by 0 mesh is anappropriate fuel. For optimum results, the bed particles should be ofnonuniform size distribution, and the fuel particles should be generallylarger than the bed particles. These and other specific parameters forpractice of this invention will easily be verified by the skilled workerin the art in accord with known design and operational characteristicsof fluidized beds.

While this invention has been described in relation to a specificoxidizing fluidized bed boiler configuration, it may be practiced in anysuitable fluidized bed boiler. Furthermore, practice of the invention isnot limited to an oxidizing fluidized bed, but will find application fora variety of reactions and reactants in fluidized beds in general. Theinvention is therefore to be limited only by the scope of the followingclaims.

I claim:

1. A method for controlling a reaction within a fluidized bedcomprising:

injecting a localized stream of fluid into the bed at a point near thebase of the bed; 7 injecting a stream of reactant into the bed near thepoint at which the localized stream of fluid is injected; injecting adistributed stream of fluid into the bed from points spaced about thelower surface area of the bed;

regulating the relative pressures and flow rates of the localized anddistributed fluid streams so that a selected fractional portion of thebed is fluidized, with at least some part of the fractional portion incontact with the stream of reactant;

maintaining the fluidized portion of the bed in a state conducive to adesired reaction; and

whereby localized reaction takes place in the fluidized fractionalportion of the bed, and the static portion of the bed remainssubstantially impervious to incursions by the fuel and localized fluidstream.

2. A method for controlling the thermal output of an oxidizing fluidizedbed comprising:

injecting a localized stream of gas into the bed at a point near thebase of the bed;

injecting a stream of fuel into the bed near the point at which thelocalized stream of gas is injected;

injecting a distributed stream of gas into the bed from points spacedabout the lower surface area of the bed;

regulating the relative pressures and flow rates of the localized anddistributed gas streams so that a selected fractional portion of the bedis fluidized, with at least some part of the fractional portion incontact with the stream of fuel;

maintaining the fluidized portion of the bed above the minimum ignitiontemperature of the fuel; and

whereby localized combustion takes place in the fluidized fractionalportion of the bed, and the static portion of the bed remainssubstantially impervious to incursions by the fuel and localized gasstream.

3. A method for controlling the thermal output of an oxidizing fluidizedbed as claimed in claim 2, in which the step of regulating furthercomprises:

regulating the pressure and flow rate of the localized stream of gas sothat the localized stream of gas is sufficient alone to fluidize afractional portion of the bed; and

regulating the pressure and flow rate of the distributed stream of gasso that the distributed stream has a minimal effect upon fluidization ofthe bed. 4. A method for controlling the thermal output of anoxidizingfluidized bed as claimed in claim 2, in which the step ofregulating further comprises:

regulating the pressure and flow rate of the localized stream of gas sothat the localized stream of gas is insufficient alone to fluidize anyfractional portion of the bed; and

regulating the pressure and flow rate of the distributed stream of gasso that the distributed stream of gas alone is insufficient to fluidizeany portion of the bed, but in combination with the localized stream ofgas is sufficient to fluidize a fractional portion of the bed.

5. A method for controlling the thermal output of an oxidizing fluidizedbed as claimed in claim 2, in which the step of regulating furthercomprises:

regulating the pressure and flow rate of the localized stream of gas sothat the localized stream of gas is sufficient alone to fluidize afractional portion of the bed; and

regulating the pressure and flow rate of the distributed stream of gasso that the distributed stream of gas alone is insufficient to fluidizeany portion of the bed, but in combination with the localized stream ofgas is sufficient to fluidize a larger fractional portion of the bedthan the localized stream of gas acting alone.

6. A method for controlling the thermal output of an oxidizing fluidizedbed as claimed in claim 2, in which the step of regulating furthercomprises:

regulating the pressure and flow rate of the localized stream of gas sothat the localized stream of gas is insufficient alone to fluidize anyfractional portion of the bed; and regulating the pressure and flow rateof the distributed stream of gas so that the distributed stream of gasalone is sufficient to fluidize only a fractional portion of the bed,and in combination with the localized stream of gas is sufficient tofluidize a larger fractional portion of the bed. 7. A method forcontrolling the thermal output of an oxidizing fluidized bed as claimedin claim 2, in which:

the steps of injecting a localized stream of gas and injecting a streamof fuel are performed by injecting a mixture of gas, such as air orfluegas, and particulate fuel, such as coal, into the bed; and

the step of injecting a distributed stream of gas into the bed comprisesinjecting a distributed stream of oxygen laden air into the bed topromote combustion within the bed.

8. A method for controlling the thermaloutput of an oxidizing fluidizedbed as claimed in claim 3 in which:

the steps of injecting a localized stream of gas and injecting a streamof fuel are performed by injecting a mixture of gas, such as air orfluegas, and particulate fuel, such as coal, into the bed; and

the step of injecting a distributed stream of gas' into the bedcomprises injecting a distributed stream of oxygen laden air into thebed to promote combustion within the bed.

9. A method for controlling the thermal output of an oxidizing fluidizedbed as claimed in claim 4 in which:

The steps of injecting a localized stream of gas and injecting a streamof fuel are performed by injecting a mixture of gas, such as air orfluegas, and particulate fuel, such as coal, into the bed; and

the step of injecting a distributed stream of gas into the bed comprisesinjecting a distributed stream of oxygen laden air into the bed topromote combustion within the bed.

10. A method for controlling the thermal output of an ox idizingfluidized bed as claimed in claim 5 in which:

the steps of injecting a localized stream of gas and injecting a streamof fuel are performed by injecting a mixture of gas, such as air orfluegas, and particulate fuel, such as coal, into the bed; and

the step of injecting a distributed stream of gas into the bed comprisesinjecting a distributed stream of oxygen laden air into the bed topromote combustion within the bed.

11. A method for controlling the thermal output of an oxidizingfluidized bed as claimed in claim 6 which:

the steps of injecting a localized stream of gas and injecting a streamof fuel are performed by injecting a mixture of gas, such as air orfluegas, and particulate fuel, such as coal, into the bed; and

the step of injecting a distributed stream of gas into the bed comprisesinjecting a distributed stream of oxygen laden air into the bed topromote combustion within the bed.

2. A method for controlling the thermal output of an oxidizing fluidizedbed comprising: injecting a localized stream of gas into the bed at apoint near the base of the bed; injecting a stream of fuel into the bednear the point at which the localized stream of gas is injected;injecting a distributed stream of gas into the bed from points spacedabout the lower surface area of the bed; regulating the relativepressures and flow rates of the localized and distributed gas streams sothat a selected fractional portion of the bed is fluidized, with atleast some part of the fractional portion in contact with the stream offuel; maintaining the fluidized portion of the bed above the minimumignition temperature of the fuel; and whereby localized combustion takesplace in the fluidized fractional portion of the bed, and the staticportion of the bed remains substantially impervious to incursions by thefuel and localized gas stream.
 3. A mEthod for controlling the thermaloutput of an oxidizing fluidized bed as claimed in claim 2, in which thestep of regulating further comprises: regulating the pressure and flowrate of the localized stream of gas so that the localized stream of gasis sufficient alone to fluidize a fractional portion of the bed; andregulating the pressure and flow rate of the distributed stream of gasso that the distributed stream has a minimal effect upon fluidization ofthe bed.
 4. A method for controlling the thermal output of an oxidizingfluidized bed as claimed in claim 2, in which the step of regulatingfurther comprises: regulating the pressure and flow rate of thelocalized stream of gas so that the localized stream of gas isinsufficient alone to fluidize any fractional portion of the bed; andregulating the pressure and flow rate of the distributed stream of gasso that the distributed stream of gas alone is insufficient to fluidizeany portion of the bed, but in combination with the localized stream ofgas is sufficient to fluidize a fractional portion of the bed.
 5. Amethod for controlling the thermal output of an oxidizing fluidized bedas claimed in claim 2, in which the step of regulating furthercomprises: regulating the pressure and flow rate of the localized streamof gas so that the localized stream of gas is sufficient alone tofluidize a fractional portion of the bed; and regulating the pressureand flow rate of the distributed stream of gas so that the distributedstream of gas alone is insufficient to fluidize any portion of the bed,but in combination with the localized stream of gas is sufficient tofluidize a larger fractional portion of the bed than the localizedstream of gas acting alone.
 6. A method for controlling the thermaloutput of an oxidizing fluidized bed as claimed in claim 2, in which thestep of regulating further comprises: regulating the pressure and flowrate of the localized stream of gas so that the localized stream of gasis insufficient alone to fluidize any fractional portion of the bed; andregulating the pressure and flow rate of the distributed stream of gasso that the distributed stream of gas alone is sufficient to fluidizeonly a fractional portion of the bed, and in combination with thelocalized stream of gas is sufficient to fluidize a larger fractionalportion of the bed.
 7. A method for controlling the thermal output of anoxidizing fluidized bed as claimed in claim 2, in which: the steps ofinjecting a localized stream of gas and injecting a stream of fuel areperformed by injecting a mixture of gas, such as air or fluegas, andparticulate fuel, such as coal, into the bed; and the step of injectinga distributed stream of gas into the bed comprises injecting adistributed stream of oxygen laden air into the bed to promotecombustion within the bed.
 8. A method for controlling the thermaloutput of an oxidizing fluidized bed as claimed in claim 3 in which: thesteps of injecting a localized stream of gas and injecting a stream offuel are performed by injecting a mixture of gas, such as air orfluegas, and particulate fuel, such as coal, into the bed; and the stepof injecting a distributed stream of gas into the bed comprisesinjecting a distributed stream of oxygen laden air into the bed topromote combustion within the bed.
 9. A method for controlling thethermal output of an oxidizing fluidized bed as claimed in claim 4 inwhich: The steps of injecting a localized stream of gas and injecting astream of fuel are performed by injecting a mixture of gas, such as airor fluegas, and particulate fuel, such as coal, into the bed; and thestep of injecting a distributed stream of gas into the bed comprisesinjecting a distributed stream of oxygen laden air into the bed topromote combustion within the bed.
 10. A method for controlling thethermal output of an oxidizing fluidized bed as claimed in claim 5 inwhich: the steps of injeCting a localized stream of gas and injecting astream of fuel are performed by injecting a mixture of gas, such as airor fluegas, and particulate fuel, such as coal, into the bed; and thestep of injecting a distributed stream of gas into the bed comprisesinjecting a distributed stream of oxygen laden air into the bed topromote combustion within the bed.
 11. A method for controlling thethermal output of an oxidizing fluidized bed as claimed in claim 6which: the steps of injecting a localized stream of gas and injecting astream of fuel are performed by injecting a mixture of gas, such as airor fluegas, and particulate fuel, such as coal, into the bed; and thestep of injecting a distributed stream of gas into the bed comprisesinjecting a distributed stream of oxygen laden air into the bed topromote combustion within the bed.